Umbran said:
True. But defense from whom? As if the British Isles themselves didn't contain anyone who'd take a pick at a community of people who have largely given up violence?
This would be my point, that the monastary system of the British isles pre-Viking period appears to have been built with the idea that all their real threats would come from their neighbors and not from the sea.
In areas which feature anti-clerical sea raiders monastaries get built on top of nasty rock formations. Which would be the pattern of Italian and Greek monastaries built during periods when Muslim raiders were a great danger.
Certainly Sea-raiding was something that the British isles had seen before. St. Patrick was carried off by Pirates as a boy, for instance, and there are some writers who claim the angles and saxons started out as sea raiders.
But I think there must have been a pretty sharp distinction between the pre- and post- Viking periods. The monastaries that recorded these raids had been in existence for some time and seemed to be totally surprised by the frequency, effectiveness, and suddenness of the raiding. Lindisfarne even rebuilt itself without any significant defenses after the first raid destroyed it. A move that indicates some basic familiarity with the situation, nothing freaky enough to cause a real behavioural change, but also no idea of what the age had coming for them.
Regardless, I imagine that there are some aspects of Viking period Scandanavian culture that you could apply to the late Roman periods.
Obsession with death in battle, farm steadings and forts as opposed to towns and villas, a slightly more empowered femine gender save in the instances of slaves or strangers, lots of gift giving, hospitality laws, magic based on secret knowledge, respect for craftsmen, and small scale but diverse agriculture. A more or less democratic society which also recognized great men and slaves. Very likely elected kings for special purposes and then got rid of them.
Most of the artifacts involving actual horned helmets come from pre-Viking scandanavia. On the same note you could throw in lots of set pieces involving bog rituals and sacrafice.